Concentrating krypton and xenon in air separation by liquid oxygen wash

ABSTRACT

In air separation, oxygen containing krypton, xenon and hydrocarbons is passed from the rectifying column to a separator where as a rising gas it is washed with a controlled downflow of liquid oxygen drawn from above the sump liquid in the column. The controlled washing permits gaseous oxygen and some methane to leave the separator top while krypton and xenon are trapped in the wash liquid which returns from the separator bottom to the column sump. Thus enriched sump liquid is withdrawn for further processing to recover krypton and xenon.

United States Patent [191 Frischbier CONCENTRA'IING KRYPTON AND XENON INAIR SEPARATION BY LIQUID OXYGEN WASH [76] Inventor: Klaus Frischbier,Mainstrasse 14,

I-lanau, Germany [22] Filed: Nov. 8, 1971 [21] Appl. No.: 196,447

[30] Foreign Application Priority Data Nov. 10, 1970 Germany P 20 55099.6

[52] US. Cl 62/41, 62/22, 62/29 [51] Int. Cl. F25j 3/02, F25j 3/03 [58]Field of Search 62/22, 23, 24, 27, 62/28, 29, 41

[56] References Cited UNITED STATES PATENTS [451 Aug. 14, 1973 Simpson62/22 Primary Examiner-Norman Yudkoff Assistant Examiner-A. F. PurcellAttorney-Paul W. Garbo [5 7] ABSTRACT In air separation, oxygencontaining krypton, xenon and hydrocarbons is passed from the rectifyingcolumn to a separator where as a rising gas it is washed with acontrolled downflow of liquid oxygen drawn from above the sump liquid inthe column. The controlled washing permits gaseous oxygen and somemethane to leave the separator top while krypton and xenon are trappedin the'wash liquid which returns from the separator bottom to the columnsump. Thus enriched sump liquid is withdrawn for further processing torecover 1,963,809 6/1934 Schuftan 62/29 krypton and enon 2,423,2747/1947 Van Nuys..... 62/22 3,596,471 8/1971 Streich 62/22 8 Claims, 2Drawing Figures PATENIED 1 3. 751 .934

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INVENTOR. KLAUS FRISCHBiER AGENT CONCENTRATING KRYPTON AND XENON IN AIRSEPARATION BY LIQUID OXYGEN WASH BACKGROUND OF THE INVENTION oxygen (X165K; Kr 120K; 90K). Therefore, in

air separation plants, they end up in the liquid oxygen. The aircontains such small proportions of these two rare gases that, first ofall, a large volume of feed air is required in order to recover ameasurable amount and, secondly, a process is required for concentratingthese slight traces in the liquid oxygen. This concentration representsthe first intermediate step toward complete separation of the kryptonand xenon from the oxygen. This invention is particularly directed tothis first intermediate step.

The recovery of krypton and xenon in commercial proportions isassociated with large air separation plants. These operate in accordancewith the known low-pressure process with air compression to about 5atmospheres; cooling down in regenerators or reversing plate exchangers;fractionation into the two main components, oxygen and nitrogen, in adouble rectifying column; recovery of the products in gaseous state atambient temperature; generation of refrigeration by expansion of a partof the process air or nitrogen under pressure.

Normally, the oxygen product is withdrawn in gaseous. form above thecondenser-reboiler which connects the two rectifying columns. Since theliquid oxygen is vaporized completely,the krypton and xenon contained inthe liquid oxygen escape with the gaseous oxygen product stream.Concentration of krypton and xenon in the liquid oxygen cannot beobtained in this manner.

In, order to keep these rare gases trapped in the liquid oxygen and,preferably, to increase their concentration, it is necessary to make achange to the process. In German Pat. No. 1,099,564, it is suggested notto withdraw the oxygen product directly aobve the reboiler but onlyafter passage through a number of rectifying trays of the low-pressurecolumn above the reboiler. The idea is to wash out the rare gasescontained in the rising gas on the two or three lowest trays by means ofthe downflowing oxygen-rich liquid and to return them to the oxygensump. This is possible because the equilibrium constant K of these raregases is considerably lower than that of oxygen. For krypton, K y/x isapproximately l 15. This means that the concentration of krypton in theliquid oxygen is about times larger at equilibrium than in the vapor.The reflux ratio of liquid to vapor on the lowest trays of the oxygensection of an air separation plant is relatively high: 1.30 to 1.45. Theliquid portion amounts to about 70 percent of the air volume.Consequently, the rare gas portion (Kr X) therein. is about 1.5 ppm.This liquid flows toward the lowermost barrier" trays. By providing asufficiently large number of rectifying trays in the barrier zone, it ispossible to achieve a nearly equilibrium condition between the vaporescaping from the barrier zone and the inpo m ingliquid; i.e. with K H15and x 1.5 ppm, y 0.] ppm. Since only parts of gaseous oxygen arewithdrawn laterally, it would be possible, by means of such a barrierzone, to trap about 98 percent of the krypton and xenon in the liquidoxygen of the lowpressure column and to concentrate it therein to aconsiderable degree. However, the simultaneously occurring hydrocarbonconcentration puts a limit on the krypton and xenon concentration.

In air separation plants, the danger stems mainly from the C through Chydrocarbons. Their solubility in liquid oxygen is relatively high, withthe exception of acetylene. It exceeds [000 ppm, compared to only 7 ppmfor acetylene. If the air separator is provided with gas phaseadsorbers, the acetylene is kept away from the columns. It may bedisregarded in the subsequent considerations. As compared to the raregases krypton and xenon, the equilibrium constants K y/): in the oxygenportion of the low-pressure column are higher by a factor of about 2 formethane, and lower by a factor of 10 and even smaller for the otherhydrocarbons. That is to say, the barrier effect" is almost the same formethane and considerably better for the other hydrocarbons, as comparedto krypton and xenon.

One tries to eliminate the danger of too high a concentration of C to Chydrocarbons in the liquid oxygen by inserting an adsorber having anadsorption capacity that even increases as the inlet concentrationrises. However, methane cannot be trapped that way.

On the other hand, due to the high liquid-to-vapor ratio, the methane isprevented from leaving the barrier zone; and of all things it is themethane which gets into the air separator at relatively the highestconcentration together with the process air. This is shown, almostwithout exception, by all air analyses. The amount is often 10 to 20times larger than that of all other hydrocarbons combined.

Although, of all the hydrocarbons, it is methane that has the highestsolubility in oxygen, it is irresponsible for safety reasons to allowthe methane volume retained within and below the barrier zone to exceeda certain kilo amount. The theoretically possible explosive power mustbe kept within limits.

The endeavour to trap krypton and xenon by means of the barrier zone andthe safety requirements for limiting the methane content in liquidoxygen are opposed to each other. For all practical purposes, themethane content therefore limits the rare gas concentration.

Therefore, in the aforementioned patent, the basic idea was weakened byproviding the possibility of withdrawing the oxygen product gas frombelow the barrier zone" from time to time. This does serve to decreaseand limit the methane level, but at the same time it creates an escapehatch" for the rare gases, krypton and xenon, through which they escapetogether with the methane. The previously enforced concentration belowthe barrier zone is lost. In case of a high methane content, it may benecessary to open the hatch so frequently that the rare gasconcentration is no longer acceptable. Another way of controlling themethane would be to keep the hatch open continuously, although throttleddown. This, however, leads to a constantly lower yield.

It is the object of this invention to overcome the described dilemma.

SUMMARY OF THE INVENTION A process has been found for concentratingkrypton and xenon during air separation by rectification wherein liquidoxygen containing krypton, xenon and hydrocarbons and drawn from thesump section of a rectifying column is vaporized, freed of krypton andxenon by rectification, and withdrawn as gaseous oxygen product.

The invention is characterized by the combination of the followingfeatures:

a. the liquid oxygen containing krypton, xenon and hydrocarbons anddrawn from the sump section of the rectifying column is conducted into avessel provided with several rectifying trays, entering below therectifying trays,

b. the liquid oxygen drawn from the sump section of the rectifyingcolumn at about the level of the lowest rectifying tray is dumped asreflux on the rectifying trays of the foresaid vessel,

c. gaseous product oxygen, freed of krypton and xenon on the rectifyingtrays, is withdrawn at the head of the aforesaid vessel, and

d. the krypton and xenon-enriched wash liquid is withdrawn from thebottom of the aforesaid vessel and returned to the sump of therectifying column while liquid oxygen enriched in krypton, xenon andsome hydrocarbons is withdrawn from the sump of the rectifying columnfor further concentration.

According to this invention, therefore, the barrier zone for the gaseousproduct oxygen is removed from the rectifying column and installedseparately.

Another essential feature of the invention is that it is possible, bymeans of the liquid oxygen drawn from the sump section of the rectifyingcolumn, to adjust the reflux ratio in the separate barrier zone," i.e.,on the rectifying trays of the separate vessel, to any desired value.Since the equilibrium constant K for methane is about twice as high as Kfor krypton, the reflux in the separate barrier zone" can be adjusted sothat methane can escape with the gaseous phase, while krypton and xenonare still being washed out of the gas stream.

As explained, the basic idea of the invention is to provide a barrier"which, due to the reflux being variable within wide limits independentlyof the air separation plant, is a barrier" in the true sense of the wordfor the rare gases, krypton and xenon, but represents a continuouslyopen hatch" for methane. As wash liquid for the separate barrier," partof the liquid running off the lowest tray of the rectifying column iscollected and withdrawn. This liquid has a lower rare gas content thanthat of the sump liquid in the rectifying column and is therefore mostsuitable as barrier liquid.

The low-pressure column of a double rectifying column serves as asuitable rectifying column. The vaporization of the sump liquid may takeplace in the known manner by condensing nitrogen from the mediumpressurecolumn in a tube condenser located within the double column between thelow-pressure and mediumpressure columns. However, it may also take placein a plate condenser of the type frequently used in modern plants. Thisplate condenser may be located outside the rectifying columns betweenthe low-pressure and medium-pressure columns; therein, sump liquid fromthe low-pressure column is heated by condensing nitrogen vapor from themedium-pressure column. In such case, the sump liquid to be vaporized ispumped through the condenser in excess in order to prevent dryevaporation.

For a fuller understanding of the invention, two illustrativeembodiments will now be explained in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a flowsheet for the two-stage rectification of air, whereinthe condenser-reboiler is located within a double rectifying columnbetween the mediumpressure and low-pressure columns; and

FIG. 2 is a partial flowsheet similar to that of FIG. 1, with a platecondenser-reboiler located outside the rectifying columns.

DESCRIPTION OF PREFERRED EMBODIMENTS:

FIG. 1 shows the known double column consisting of medium-pressurecolumn 1, low-pressure column 2 and condenser-reboiler 3 located betweenthe two columns. The air to be separated enters the plant through line4, heats the sump liquid of medium-pressure column 1 by means of heatingcoil 5 and discharges into mediumpressure column 1 through line 6. Theresultant crude liquid oxygen enters low-pressure column 2 through line7 and the liquid nitrogen through line 8. From lowpressure column 2,pure nitrogen is withdrawn through line 9, while liquid oxygen enrichedwith krypton and xenon accumulates in the sump. This sump liquid isvaporized by gaseous nitrogen in medium-pressure column l whichcondenses in condenser-reboiler 3.

Due to the high liquid-to-vapor ratio (1.30 to L45) in the lower portionof rectifying column 2, the entire krypton and xenon content of thegaseous phase is washed down out of the rising gas. A part of this gasis withdrawn as product oxygen above condenserreboiler 3 and conductedinto separator 12 through line 11. In separator 12, it flows up throughseveral rectifying trays 13 on which liquid oxygen is dumped fromchannel 10 in column 2 by pump 14 through line 15. The liquid volume ofthis reflux is selected so that a liquid-to-vapor ratio between 0.05 and0.2, preferably 0.1, is created. This reflux is capable of washing outthe krypton and xenon present in the rising oxygen gas, whereas themethane also present in the oxygen gas partially remains in the gaseousphase due to the low liquid-to-vapor ratio.

The reflux ratio may be adjusted so that the amount of methane leavingthe air separation plant with the gaseous product oxygen ls equal to theamount that enters the plant with the process feed air. In this way, acertain methane level will establish itself in the liquid oxygen.

If the methane level exceeds the given limit, the reflux ratio isreduced so that a lower level establishes itself in the liquid oxygen. Aloss of rare gases will occur only if the reflux ratio is reduced to avalue which is lower than that required for retaining the rare gases;i.e., the liquid-to-vapor minimum is equal to K y/k l/l5 0.066.

The gaseeus product oxygen, thus freed of krypton and xenon, then leavesseparator 12 through line 17.

Thus, it is possible to keep the krypton and xenon trapped in the liquidoxygen accumulating as a pool in separator 12 and passing thence throughline 18 and valve 16 back into the sump of column 2. As the plantcontinues operating, the amount of trapped krypton and xenon willincrease. Through line 19, a part of the crude concentrate is thenwithdrawn as a liquid for further processing.

FIG. 2 shows a variation of the invention wherein heating of the sumpliquid of the low-pressure column takes place in an aluminum platecondenser-reboiler located outside both rectifying columns, as is oftenthe case in modern air separation plants. Similar plant components aremakred in FIG. 2 with the same reference numerals used in FIG. 1.

Low-pressure column 2 has lines 7 and 8 which, respectively, feed liquidcrude oxygen and liquid nitrogen from the medium-pressure column whichis not shown. Part of the liquid oxygen containing krypton and xenon,which accumulates in the bottom of column 2, is withdrawn by pump andconducted into plate condenser-reboiler 22 through line 2i. There, theliquid oxygen is vaporized by condensing gaseous nitrogen from themedium-pressure column which enters through line 23 and leaves as liquidthrough line 24 and then is dumped in part into the top of low-pressurecolumn 2 through line 8; the remainder of the liquid nitrogen leavingthrough line 24 returns to the top of the medium-pressure column asreflux. The liquid oxygen is pumped through plate condenser-reboiler H2in excess to prevent dry evaporation. Consequently, a liquid and vapormixture enters separator 12 through line 25. The gas rises throughrectifying trays I3 countercurrent to the wash liquid which pump 24withdraws through line 15 from channel 110 located at the level of thelowest rectifying tray in the bottom section of rectifying column 2.Krypton and xenon are washed into the liquid accumulating in the bottomof separator 12 and are returned therewith to the sump of rectifyingcolumn 2 through line 18 and valve 16 which is used to control theliquid level. The greater part of the hydrocarbons, particularly themethane, is not washed out on the rectifying trays in separator l2 dueto the selected low liquid-to-vapor ratio, and leaves separator 112through line R7 together with the gaseous product oxygen. A partialstream of the gaseous phase separated in separator 12 is not conductedup through rectifying trays 113 but is withdrawn through line 26 andreturned into the bottom of low-pressure column 2. This partial streamprovides the necessary reboil vapor for lowpressure coumn 2. Thekrypton, xenon and hydrocarbons in this partial stream are washed outcompletely on the rectifying trays of low-pressure column 2 and end upin its sump. Again, as in the process of FIG. ll, a relatively highconcentration of krypton and xenon in the liquid oxygen in the sump oflow-pressure column 2 is obtained before the rising hydrocarbon contentputs an end to any further concentration of krypton and xenon. Theconcentrate is withdrawn through line 19 and subjected to furtherconcentration processes which are known and do not form a part of thisinventron.

The relatively high krypton and xenon concentration obtained in thefirst concentration by this invention reduces the cost of furtherconcentration. Since a large percentage of the hydrocarbons leave theplant through line 117 together with the gaseous product oxygen, theknown process for removing hydrocarbons from the krypton and xenonconcentrate becomes simpler and cheaper as well.

What is claimed is:

H. In the process of separating oxygen and nitrogen in an air rectifyingcolumn, the improvement of recovering a concentrate of krypton and xenonwhich comprises discharging into the top of a krypton and xenonconcentrating rectifying column liquid oxygen containing smallquantities of krypton, xenon and hydrocarbons drawn from a level nearthe bottom tray of said air rectifying column before said liquid oxygenflows into the sump of said air rectifying column, passing a stream ofoxygen containing increased quantities of krypton, xenon andhydrocarbons from below the bottom tray in said air rectifying column tobelow the bottom tray in said concentrating rectifying column, adjustingthe discharge of said liquid oxygen into the top of said concentratingrectifying column to provide a liquid-tovapor ratio at which krypton andxenon are substantially completely washed out of the vapor risingthrough said concentrating rectifying column, withdrawing gaseous oxygencontaining some of said hydrocarbons but substantially free of kryptonand xenon from the top of said concentrating rectifying column,returning liquid from the sump of said concentrating rectifying columnto the sump of said air rectifying column, and recovering liquid fromthe sump of said air rectifying column as said concentrate of kryptonand xenon.

2. The process of claim 1 wherein the stream of oxygen containingincreased quantitics of krypton, xenon and hdyrocarbons from below thebottom tray in the air rectifying column is liquid from the sump of saidair rectifying column, said liquid is partially vaporized before beingpassed to below the bottom tray in the concentrating rectifying column,and part of the vapor of the partially vaporized liquid is passed frombelow the bottom tray in said concentrating rectifying column to belowthe bottom tray in said air rectifying column.

3. The process of claim 1 wherein the adjustment of the discharge ofliquid oxygen into the top of the concentrating rectifying colomnprovides a liquid-to-vapor ratio in the range of about 0.05 to 0.02.

4. The process of claim ll wherein the air rectifying column is thelow-pressure column of a double rectifying colomn, and vapor in the topof the mediumpressure column of said double rectifying column indirectlytransfers heat to liquid in the sump of said lowpressure column.

5. The process of claim 4 wherein the adjustment of the discharge ofliquid oxygen into the top of the concentrating rectifying columnprovides a liquid-to-vapor ratio in the range of about 0.05 to 0.2.

6. The process of claim 5 wherein the indirect transfer of heat fromvapor to liquid is conducted outside the low-pressure andmedium-pressure columns and effects partial vaporization of said liquid,the partially vaporized liquid is passed to below the bottom tray in theconcentrating rectifying column as the stream of oxygen containingincreased quantities of krypton, xenon and hydrocarbons, and part of thevapor of said partially vaporized liquid is passed from below the bottomtray in said concentrating rectifying column to below the bottom tray insaid low-pressure column.

7. In an air rectifying plant wherein liquid oxygen collects in the sumpof a rectifying column and is reboiled, the improvement of means forconcentrating krypton and xenon in said liquid oxygen in said sump whichcomprises an auxiliary rectifying column, first means for drawing liquidfrom a level just below the bottom tray in said rectifying column andintroducing said liquid on the top tray in said auxiliary rectifyingcolumn, second means for passing a stream drawn from a lower level belowsaid bottom tray to below the bottom tray in said auxiliary rectifyingcolumn, an outlet at the top 8. The air rectifying plant of claim 7wherein the first of said auxiliary rectifying column for gaseousoxygen, means for drawing liquid includes a pump with a variand a valvedpipe connecting the bottom of said auxilable pumping rate.

iary rectifying column to said sump.

UNITED STATES PATENT oFTTcE @ERTIFICATE 0H QURREQTWN Pa n s vsl sa DatedAugust m 19% Inventor (s) Klaus Frischbier It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown Below:

Column 1, line 45, correct "eobve" to W above Column 5, line 6, ea-Tract"makred" to marked line 22, correct "12" to e 22 '""&

' Column 6, line 56, correct "0002 to w 00,2 o

Signed and sealed this. 20th day of November 1973.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. RENE D.. TEGTI IEYER Attesting Officer 1 ActingCommissioner of Patents F ORM PC4050 (10-69 USCOMM-DC 60376-P69 I U.SGOYERNMENT IRIN TING QfFICE I969 0-366-3 f

2. The process of claim 1 wherein the stream of oxygen containingincreased quantities of krypton, xenon and hdyrocarbons from below thebottom tray in the air rectifying column is liquid from the sump of saidair rectifying column, said liquid is partially vaporized before beingpassed to below the bottom tray in the concentrating rectifying column,and part of the vapor of the partially vaporized liquid is passed frombelow the bottom tray in said concentrating rectifYIng column to belowthe bottom tray in said air rectifying column.
 3. The process of claim 1wherein the adjustment of the discharge of liquid oxygen into the top ofthe concentrating rectifying colomn provides a liquid-to-vapor ratio inthe range of about 0.05 to 0.02.
 4. The process of claim 1 wherein theair rectifying column is the low-pressure column of a double rectifyingcolomn, and vapor in the top of the medium-pressure column of saiddouble rectifying column indirectly transfers heat to liquid in the sumpof said low-pressure column.
 5. The process of claim 4 wherein theadjustment of the discharge of liquid oxygen into the top of theconcentrating rectifying column provides a liquid-to-vapor ratio in therange of about 0.05 to 0.2.
 6. The process of claim 5 wherein theindirect transfer of heat from vapor to liquid is conducted outside thelow-pressure and medium-pressure columns and effects partialvaporization of said liquid, the partially vaporized liquid is passed tobelow the bottom tray in the concentrating rectifying column as thestream of oxygen containing increased quantities of krypton, xenon andhydrocarbons, and part of the vapor of said partially vaporized liquidis passed from below the bottom tray in said concentrating rectifyingcolumn to below the bottom tray in said low-pressure column.
 7. In anair rectifying plant wherein liquid oxygen collects in the sump of arectifying column and is reboiled, the improvement of means forconcentrating krypton and xenon in said liquid oxygen in said sump whichcomprises an auxiliary rectifying column, first means for drawing liquidfrom a level just below the bottom tray in said rectifying column andintroducing said liquid on the top tray in said auxiliary rectifyingcolumn, second means for passing a stream drawn from a lower level belowsaid bottom tray to below the bottom tray in said auxiliary rectifyingcolumn, an outlet at the top of said auxiliary rectifying column forgaseous oxygen, and a valved pipe connecting the bottom of saidauxiliary rectifying column to said sump.
 8. The air rectifying plant ofclaim 7 wherein the first means for drawing liquid includes a pump witha variable pumping rate.